In an effort to reduce weight and volume, especially in aircraft applications where many systems are actuated during aircraft operation, it has been conventional to utilize a single motor to drive two separate but related actuation systems sequentially. For instance, take the case of cargo bay doors and a pallet loaded and unloaded, sequentially into the cargo bay. The motor will open the cargo bay doors and then lower the cargo pallet after which the motor reverses to raise the loaded or unloaded cargo pallet as the case may be and then close the cargo bay doors. Because the motor has to coordinate two separate but related functions, there has been a necessity to provide a timing mechanism for shifting from the operation of one system to the operation of the other system. It has been found that over the passage of time, such mechanisms develop slip which creates problems of mistiming of the two systems.
In those applications where one or both of the systems has to be operated rapidly, friction devices are sometimes provided in conventional systems to slow the system down from full speed toward the end of its stroke. The presence of such friction devices can further distort the positional relationships between the two systems in relation to the motor with the result of less and less accurate timing.